An ultrasonic motor is a motor using rotational torque via a friction force which is applied to a rotor in press-contact with a stator of a piezoelectric vibrator generating an ultrasonic elliptic vibration. Compared to a conventional electromagnetic motors based on the interaction between the electric current and magnetic fields, it has such entirely different features as low speed and high torque. More particularly, the torque per unit volume is large, and the ultrasonic motor can be driven without gears and brakes if it is used at low speed.
Various ultrasonic motors have been proposed and researched. For instance, a traveling wave type ultrasonic motor using a traveling wave of bending which is generated on the circumference of a stator is described in Japanese Patent Kokai No. 58-148682. In the traveling wave type ultrasonic motor, the traveling waves which are traveling chronologically are excited on the stator which comprises a piezoelectric ceramic element to rotate a rotor in press-contact tightly with the surface of the stator. However, it is difficult to obtain high torque if the diameter of the ultrasonic motor is small, as the ultrasonic motor uses bending vibration. For instance, torque of a traveling wave type ultrasonic motor having a diameter of 2 cm is no more than 0.1 to 0.2 kgf cm.
On the other hand, a standing wave type ultrasonic motor using a standing wave of bending which is generated on the circumference of a stator is described in Japanese Patent Kokai No. 61-52163. In the standing wave type ultrasonic motor, an ultrasonic elliptic vibration is generated efficiently at an interface between a rotor and a stator. However, the arrangement of structural design elements of the ultrasonic motor is limited, in other words, the degree of freedom in design of shape and size of a vibrator is limited, as it needs a mode conversion in order to excite a torsional vibration by using a piezoelectric longitudinal vibration. Furthermore, the rotating direction, of the elliptic vibration is dependent on the structural design, so that the ultrasonic motor cannot be changed in the rotating direction of the rotor freely.
The inventors have proposed an improved ultrasonic motor, in which a rotating direction of a rotor can be changed freely, having a small diameter and a high torque, as described in Japanese Patent Application No. 62-149726 and on pages 821 to 822 of "Autumn Term Research Meeting Record No. 2-4-10, Oct. 1988, The Acoustical Society of Japan". The ultrasonic motor has a stator consisting of a longitudinal and torsional composite vibrator, having a small diameter and a high torque, and can be changed in the rotating direction of a rotor freely. In the ultrasonic motor, longitudinal and torsional vibrations are excited simultaneously as a resonance vibration to obtain an elliptic vibration which is a composite vibration of the longitudinal and torsional vibrations efficiently and intensively in the interface of the stator and the rotor. In this case, it is necessary that the resonance frequencies of the longitudinal and torsional vibrations are equal in order to excite the resonance vibration. In this ultrasonic motor, the equality of the resonance frequencies of the longitudinal and torsional vibrations is realized in a state that a small electric field is applied. In this state, the strength of the contact-pressure between the rotor and the stator is adjusted by use of a shaft having an adequate diameter which is mounted vertically on the stator.
According to the conventional ultrasonic motor, however, there is a disadvantage in that it is difficult to equalize the resonance frequencies of the longitudinal and torsional vibrations in a state that a large electric field is applied in practical use, for the reason that the resonance frequency of the torsional vibration becomes higher than that of the longitudinal vibration in a large electric field.